The invention relates to processes and devices for gluing dried fibers designated for the production of fiberboards. The fibers preferably consist of lignocellulose-containing and/or cellulose-containing materials. The fiberboards are light, medium-density or high-density fiberboards.
It is conventional to glue fibers, which are designated for the production of MDF- or HDF-boards, in the wet state. By means of this so-called blow-line gluing system, the binding agent is sprayed into a blowpipe, which terminates in the inlet region of the pipe drier, behind a refiner and on to the wet, still hot fibers. The fibers are then dried. The blow-line gluing system renders it possible to glue fibers in a uniform manner and thus to obviate the formation of lumps made up of glue and fibers. However, one considerable disadvantage of the blow-line gluing system is the relatively high glue consumption (cf. e.g.: Buchholzer, P., “Leimverlusten auf der Spur” [on the track of glue losses], pages 22–24, MDF-Magazin 1999). The increased glue consumption is caused by virtue of the fact that a part of the reactivity of the glue during the process of drying the fibers is lost by reason of the high temperatures. Therefore, in the dryer system the emission of formaldehyde, which originates from the glue, is considerable, whereby it is necessary to implement a costly process of minimizing pollutants. A further disadvantage of the blow-line gluing system is that the fibers which are glued in this manner have a low level of cold-stickiness due to the pre-curing in the dryer, so that after preliminary pressing a fiber mat which is formed from the fibers has a high spring-back tendency. During compaction of the fiber mat, this can cause the fiber mat structure to be destroyed by reason of a substantial displacement of air from the fiber mat.
The disadvantages of the blow-line gluing system can also be avoided by gluing the fibers in the dry state. It is thus known to glue dried fibers in a mixer. However, the process of dry-gluing fibers in mixers has the disadvantage that fiber agglomerates and matted fibers are produced which lead to non-uniform gluing of the fibers and to an undesired formation of glue spots on the surfaces of the boards (cf. ibid.). A dry gluing machine, in which mixing tools can be provided, is described in EP 0 744 259 B1.
EP 0 728 562 A2 discloses a process of dry-gluing fibers, wherein the fiber flow is separated in a pneumatic delivery line by the generation of substantial turbulence by reason of the reduced flow rate and the fibers in this separation zone are wetted by spraying.
DE 199 30 800 A1 describes a process of dry-gluing fibers, wherein the gluing process is performed in an end section of a pipe dryer. In our view, there is still no evidence of experience of this process in an industrial trial. The disadvantage of this process appears to be that an extremely large proportion of hot gas and water vapor together with the fibers must pass through the gluing zone, as it is absolutely necessary for the glue to be atomized to the smallest particles upon being sprayed into the gluing zone. In the case of this proportion of hot gas and water vapor which in the process is separated from the fibers immediately after the gluing process by means of a cyclone, it is to be assumed that a portion of the glue escapes into the atmosphere together with the hot gas and the water vapor from the fiber mixture. Furthermore, in the case of this known process, problems can arise in relation to the uniformity of the gluing in view of the random air turbulence generated. Furthermore, in the case of this process it appears to be difficult to keep the drying moisture of the fibers under control within the tolerances of +/−0.5% of the desired value which is very important for the further process.
It should also be mentioned that gluing devices of the so-called “roller blender” type have been known for some time, wherein glue is applied to wood particles by means of rollers (Maloney, Thomas M., “Modern Particleboard & Dry-process Fiberboard Manufacturing”, page 439 f., Miller Freeman Publ. 1977, San Francisco, Calif., USA).